Understanding the Earth's Atmosphere: Composition and Dynamics

1. Introduction to the Atmosphere

2. Composition and size • Gases (and water droplets and aerosols) • mechanically combined • 99.99997 % below 60 miles (100 km) • Diameter of earth? • 8000 mi (12800km) • 60/8000 ? • 0.75%

4. Average Compositionof Dry Atmosphere below 15 mi. : 99.98%

5. Other constituents: • Water vapor (and very small, suspended droplets of water) • Aerosols (non-gas): suspended particles of sea salt, dust, organic matter, smoke

6. Permanent vs. Variable Gases Most are mixed in constant proportions in bottom 50 miles (homosphere). PERMANENT GASES Exceptions are not found in constant proportions throught the homosphere or vary through time: VARIABLE : O3, H2O, CO2, aerosols

7. 1. OZONE O3 • Concentrated at 9-22 mi (15- 35 km) • “good ozone” • Stratosphere-absorbs UV light (“Bad ozone” is in lower atmosphere in minute amounts and is a pollutant).

8. UV O2 + O2 O3 O O + O2 O3 above 22 mi:low density means collisions less likely; below 9 mi : not as much UV because it has been absorbed

9. 2. Water vapor 4% at surface; almost absent > 6.2 mi (10 km ) • Source is Surface; carried upward by turbulence • most effective < 6.2 mi

10. CLOUDS WATER VAPOR Abundant water vapor; even where there are no clouds

11. 3. Carbon dioxide Varies temporally; increase in late 20th and early 21st Century due to anthropogenic production

12. 4. Aerosols Vary with source regions Sources include: factories, urban areas, volcanic eruptions

13. Review • Atmosphere is mixture of gases and aerosols • Vast majority is below 15 mi • Predominantly N2 and O2 • All mixed in same proportion • Except variable: O3 H2O, CO2, aerosols

14. Atmosphere obeys mechanical laws because it has mass. behaves like a single“ideal gas”.

15. “Ideal gas” • Follows kinetic molecular theory; gas is made up of many molecules in rapid, random motion. • Perfect elastic collisions; so no momentum is lost in collisions. • Small enough that attractive forces between them are negligible.

16. Pressure, density, temperature and volume are interrelated. Temperatureis average speed at which molecules are moving in a gas. Pressureis force per area. Densityis mass per volume.

17. Gas pressure (different than atmospheric pressure) Balloon analogy: Gas pressure is caused by impacts of molecules on inside of balloon. The greater the number of collisions, the greater the pressure (force per unit area).

18. Pressure and volume (& DENSITY)(If temperature is constant) If volume (of balloon) is increased, density decreases, pressure _?_. Pressure and density are directly proportional.

19. At the same temperature, air at a higher pressure is more dense than air at a lower pressure.

20. Pressure and temperature(If density is constant) If temperature increases, molecular movement becomes more vigorous, and pressure __. Pressure and temperature are directly proportional.

21. Temperature and density(If pressure is constant) If temperature increases, gas expands, volume increases, density decreases. Temperature is inverselyproportional to density. Summary of relationships: Pressure ~ Density x Temperature P ~ ρ x T

22. Ideal Gas Law Equation: temperature density P = ρRT pressure Gas constant , for dry air = 287 J kg-1 K-1

23. P =ρ x R x T • At a given pressure, air that is cold is more dense than air that is warm. • At a given density, increasing temperature increases molecular movement, which increases pressure. • At a given temperature, increasing density increases pressure by increasing number of collisions.

24. Vertical structure of atmosphere: • Atmospheric molecules have mass. • Earth exerts gravitationalforce on gas molecules in atmosphere. Results: • Air is compressible. • Mass, density and pressure change with height.

25. Recap… • Pressure, Density, temperature are related: P ~ ρ T • Atmosphere is compressible because it has mass. • Atmosphere is layered

26. Atmospheric pressure vs. gas pressure Atmospheric pressure on the outside of the balloon is due to impacts from molecules on outside. Amount of pressure is due to density of atmosphere. Density decreases with height.

27. Atmospheric pressure: • force exerted by a column of air per unit area • at sea level: • 14.7 lb/in2 • 1.034 kg/cm2 • 1013 millibars ( 1 mb = 100 Pa) 14.7 lb 1.034 kg

28. 29.92 inches of Mercury (Hg)

29. Mass (proportional to weight) 50% mass of atmosphere : below 3 mi 90% below 10 mi • Density (mass per volume): 1.2 kg m-3 at surface 0.7 kg m-3 at 3 mi • Pressure (force per unit area): • Atmospheric pressure (vs. gas pressure) At 3 mi from surface, pressure drops by half

30. Realtionship negative exponential decrease

32. Due to these changes… …atmosphere is stratified. “vertical structure” of the atmosphere (layers)

33. Profile of temperature change with altitude Temperature Layers of Atmosphere

34. Review • Pressure, density, mass decrease exponentially with altitude. • Atmosphere is therefore “layered”. Troposphere • Temperature decreases with height; weather; 90% mass • Altitude of tropopause varies with latitude Stratosphere Temp increases with altitude; ozone; stratified Mesosphere Temp decreases with height; lowest temp at stratopause “shooting stars”, noctilucent clouds Thermosphere Temp increase with altitude Auroras

35. Earth has North and South magnetic poles and a magnetic field

36. Magnetic poles not same as rotational poles

38. “Solar Wind” • Sunspot activity associated with appearance of auroras. • Solar Wind : Ionized particles streaming from sun towards earth. • Solar wind has magnetic fields.

39. Solar wind distorts earth’s magnetic field (magnetosphere)

40. Magnetic fields in solar wind disturbs magnetosphere. • Magnetic field funnels electrons into upper atmosphere. • Particles in magnetosphere enter thermosphere and excite atoms to emit visible radiation.

41. Line is region of most likely occurrence, Northern Hemisphere

43. Line is region of most likely occurrence, Southern Hemisphere